This invention relates generally to semiconductor lasers and, more particularly, to a continuous wave optically pumped tunable semiconductor laser having an external resonant cavity.
Lasers exist in many shapes and forms, yet the search for new types of lasers continues unabated. Lasers vary greatly in many aspects such as, for example, power, operating wavelength, cavity design, method of pumping and mode discipline (mode-locking, single-frequency, or chaotic operation). The single, most frequent means of laser identification is by the type of gain medium utilized within the laser, since the medium will strongly influence, if not dictate, the other considerations of laser design.
Optically pumped semiconductor lasers are of especially great interest because of their potential for becoming a convenient, tunable, coherent source of electromagnetic radiation throughout the visible and near IR range of the spectrum. The most distinguishing feature of a semiconductor laser is that it does not deal with gain centers (atoms, ions, molecules, complexes) sparsely distributed in a passive medium or empty space, but rather with the phenomena of inverting the atoms in an entire block of solid, unlike any other kind of laser. Since the absorption and gain lengths in a semiconductor laser are very small compared to other lasers, this central fact greatly influences the choice of pumping scheme: active medium, heatsinking, cavity design, and sample geometry. Furthermore, semiconductors are crystals and their ordering implies spatial anisotropy (selection rules) and polarization, since polarization effects generally depend on crystal orientation.
A single bulk semiconductor is the simplest amplifying medium since it is cheap, readily available, and requires far less processing than heterostructure. To date, most optically pumped semiconductor lasers have used either crystal faces or closely attached mirrors as the cavity reflectors; this, unfortunately, prevents the insertion of tuning elements into the cavity and lowers its optical quality. Furthermore, because of high threshold pump powers and severe heating problems, semiconductor lasers in use today involve short pump pulses.
Additionally, most semiconductor crystals must be cooled to liquid nitrogen temperatures or below. Other problems arise with prior semiconductor lasers since a threshold of roughly 100 KW/cm requires a very tight beam focus for cw or quasi-cw lasing because the total power demanded by a larger spot size would destroy the crystal used therewith. The small spot size is also required to eliminate amplified spontaneous emission.
As is clearly evident from the description above, semiconductor lasers, and, in particular, optically pumped semiconductor lasers although potentially highly desirable currently have numerous drawbacks which render them less than effective under certain circumstances. It would be extremely beneficial to produce an optically pumped semiconductor laser which does not fall victim to the above-mentioned shortcomings.